This invention relates generally to the manufacture of optical parts and, more particularly, to methods for forming wafers into a complex shape that complements the curvature of an optical part into which the formed wafer is incorporated.
Good optical performance is essential for high-quality prescription and non-prescription eyewear, but several other factors frequently can affect the choice of lens design and materials. For instance, lightweight materials might be important for wearer's comfort and convenience. Fashion consideration might dictate lens shapes, such as highly curved “wrap-around” designs, and lens colors. Similarly, lenses having specific optical attributes might be desirable. For instance, lenses that are polarized are specifically designed for effective attenuation of reflected glare. This can be particularly important for better visibility in bright, snowy, hazy or wet conditions. Thus, many factors should be considered when designing and producing high-quality eyewear.
One approach to producing a lightweight polarizing lens combines a polarizing wafer, which has polarizing material sandwiched between protective layers, and thermoplastic material such as polycarbonate. One representative manufacturing technique involves melting or fusing the thermoplastic polycarbonate with the wafer, via an injection-molding process. To provide the power—and lack of distortion—necessary for good vision, lenses must have precisely curved shapes. It is therefore essential that manufactured lenses exactly replicate the shape of the desired mold.
Exact replication can be a manufacturing challenge for any lens production, but the challenge becomes even more difficult when the lens incorporates a wafer that must assume a controlled and complex shape. One approach to avoid this difficulty in the past has been to embed the wafer deeper into the optical part, below the complex curved outer surface, such that the wafer may be flat or simply curved while the outer surface has the complex curvature needed for the correct optical function. However, this approach can limit how thin a part can be made and therefore compromise its cosmetic appearance as well as increase its weight. In addition, the optical performance of an embedded wafer may not be as efficient or effective as the performance of a wafer positioned at the optical part's outer surface. For instance, if the wafer has specific reflectance characteristics, they might be adversely affected by being embedded within the optical part due to differences in optical refraction.
Another approach has been to place a planar wafer directly against the molding surface for the optical part. However, if this molding surface has a significantly different curvature from that of the planar wafer, the wafer may not fit or replicate this surface properly, resulting in an optical distortion or incorrect optical power. In addition, the mismatch in shapes can cause irreparable damage to the wafer such as wrinkling, buckling, or burning from inconsistent thermal contact, again causing optical and cosmetic defects in the final optical part.
Pre-forming the wafer to a closer approximation of the desired mold shape might aid in this replication. For straightforward spherical lenses, several techniques have been employed to curve wafers to spherical shapes, including (1) heating and vacuum deforming into an open cavities, and (2) heat- and pressure-forming with matched molding surfaces, as described in U.S. Pat. No. 5,434,707, which is incorporated herein by reference. Vacuum deforming into an opening will only generate a smooth catenary shape, not one having a highly varying curvature. However, while a wafer having a spherical or simple catenary curvature might better approximate a complex shape than does a wafer that is planar, the wafer still might not be sufficiently curved to avoid mismatches that can lead to wrinkling, buckling, or other deformations and damage that contribute to optical distortion and aberration in the final optical part. Such mismatches can be especially problematic when trying to replicate highly asymmetrical shapes, including decentered wrap designs, and multifocal stepped or progressive designs.
Further, although one advantage of using a wafer is that it has a protected, stable construction, the extra thickness of materials associate difficulties in further re-shaping. These difficulties increase as more complex shapes are attempted. An analogy is to compare the relative ease of gift-wrapping a shoebox, as compared to the challenge of gift-wrapping a curved vase. If the wrapping material is cardboard, it obviously will be extremely difficult to accurately contour to the shape of the vase.
It should, therefore, be appreciated that there is a need for an improved method for forming wafers into complex curved shapes, for later incorporation into optical parts having complementary shapes. The present invention fulfills this need and provides further related advantages.